This allows manual triggering of the workflow via the github UI.

Enable GitHub actions to generate PDF

Add Makefile to specify actions to take in the environment.

Preserve PDF output as an artifact so that it's available to users.

Environment dependencies are recorded in package-manager-specific files
in dependencies/.

Work around issue #8.

Future possible enhancements:
  - Add a release workflow.
  - Move to a container for the environment.  This allows the
    environment description to not be local to github actions, usable
    for local development, etc.

adding note on asciidoctor-mathematical limitations and output to svg for math expressions in build command

The requirements for asciidoctor-mathematical include gdk-pixbuf (spelling fixed) and cmake.